There are a number of processes for which urea gasified by a thermal process is useful if the temperature of the gases is sufficient to permit its use without causing condensation of solids in the system. For low-temperature processing, however, the decomposition products in these gases can cause problems. See, for example: Modern Power Systems, “Ammonia SCR performance from a urea-based system”, May 2004, pages 27, 29, 30 and 31, which notes that tests showed that urea decomposition products were found to reform urea when cooled, or that they could deposit on cool surfaces as urea. They found that appropriate heating or insulation was required to obviate low-temperature surfaces. Thus, low-temperature use of the thermally-gasified urea can cause problems.
When aqueous urea is heated, a number of chemical reactions, controlled by temperature-dependent rate constants, determine how urea is broken down:
This reaction can occur at a temperature of 275° F.; but the HNCO, unless hydrolyzed or maintained very hot can form solid byproducts that can deposit on equipment and foul catalysts. The HNCO will be converted as follows:
Cyanuric acid, if formed (and it is likely to form) decomposes at about 700° F. The full conversion of urea to ammonia can involve the following reactions, but not all are desirable and efforts should be made to moderate or eliminate their negative effects:
HNCO+NH2—CO2—NH2→BiuretHNCO+Biuret→TriuretTriuret→Cyanuric Acid+NH3 3HNCO→Cyanuric Acid2NH2—CO2—NH2+H2CO→Methylene DiureaThese reactions are rate dependent as well as dependent on the physical form of the reactants, the prevailing temperature, the time in the reactor and the presence or absence of water and/or a catalyst.
There are a number of references that discuss converting urea to ammonia; however, a review of the art has not enabled the efficient conversion of urea to ammonia in a form that could be used for low-temperature operations. Prominent among the prior art processes are: (a) wet processes, such as U.S. Pat. No. 6,077,491 to Cooper, et al., and U.S. Pat. No. 5,543,123 to Hofmann, et al.; (b) high-temperature processes such as U.S. Pat. No. 7,090,810 to Sun, et al., or U.S. Pat. No. 7,682,586 to Harold, et al., and (c) catalytic processes such as, for example, U.S. Pat. No. 6,878,359, to Mathes, et al., and EP 487 886 to MAN.
Also of note for their lack of teachings enabling efficient production of ammonia from urea for low temperature operations is U.S. Pat. No. 5,431,893, to Hug, et al. To protect the SCR catalyst from fouling, Hug, et al., proposes bulky equipment capable of treating all effluent with urea. Regardless of physical form, urea takes time to break down in hot exhaust gases and may cause nozzle plugging at the temperatures most conducive to gasification. This disclosure highlights the problems making it a necessity that the urea solution is maintained at a temperature below 100° C. to prevent hydrolysis in the injection equipment. They propose the use of moderate urea pressures when feeding the urea and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged. The nozzles employed by Hug, et al., use auxiliary air to aid dispersion. Also, they employ dilute solutions that require significant heating to simply evaporate the water. See also, WO 97/01387 to Müller, et al.
In European Patent Specification 615,777 A1, there is described an apparatus that feeds solid urea into a channel containing exhaust gases, which are said to hydrolyze the urea in the presence of a catalyst. For successful operation the disclosure indicates that it is necessary to employ compressed air for dispersion of fine solids, means for grinding the urea into fine solids and a coating to prevent urea prills from sticking together. The disclosure notes that if the inside of the catalyzer and the nozzle tip only were coated with the catalyst, corrosion and deposition would occur. The introduction of solid urea into the gas stream—possibly depositing urea on the SCR catalyst—also eliminates control of water to the reactor in amounts necessary for efficient hydrolysis, without which HNCO will remain and potentially harmful byproducts will be present.
U.S. Pat. No. 6,878,359, to Mathes, et al., describes a single stage process using a catalyst to gasify urea, but provides no indication that separating gasification from hydrolysis into two stages as found highly effective for low-temperature applications by the invention herein, would be a useful alternative to a single stage process. We note that Mathes, et al., does not teach high enough initial temperature, temperature maintenance, or proper droplet size for a two stage process. Importantly, unless the droplets are small enough in the first-stage gasification, the droplets will not release the urea for decomposition early enough in a short, e.g., 1 to 10 second, time frame to fully gasify the urea, and the likelihood of forming byproducts downstream in the ductwork or the catalyst is increased.
Similar to the above U.S. Pat. No. 6,077,491 to Cooper, et al., is U.S. Pat. No. 6,146,605 to Spokoyny, where there is described a combined SCR/SNCR process in a staged process involving a separate step of hydrolyzing the urea prior to an SCR stage. A similar process is disclosed in U.S. Pat. Nos. 5,985,224 and 6,093,380 to Lagana, et al., which describe a method and apparatus involving the hydrolysis of urea followed by a separation of a gas phase from a liquid hydrolysate phase. In all these processes there is a requirement to handle a significant amount of high temperature and high pressure gas and liquid phases containing ammonia during and after hydrolysis. This extra processing requires the purchase and maintenance of additional equipment, an emergency plan and equipment to handle ammonia release in case of process failures, and it would be desirable to have a system which operated more safely, simply and efficiently.
It becomes apparent to the skilled worker that the art is not enabling for low-temperature effective ammonia from urea generation in an efficient manner. In the case of air pollution control, examples of low-temperature processing where it would be desirable to use ammonia from a urea source include flue gas conditioning. Here, a small amount of ammonia is injected, which differs from selective catalytic reduction systems (SCR) which operate at somewhat higher temperatures and depend on ammonia in relatively large amounts.
While it is noted that EP 0 373 351 to ENEL employs urea to create ammonia to enhance the efficiency of the electrostatic precipitator, the urea is supplied as a mixture of urea, hydrate lime and water for reducing pollutant materials in the flue gases and does not produce the ammonia suitable for low-temperature operations apart from the combustor. Urea reduces the NOx and hydrate lime reduces the sulfur compounds.
There is a present need for a process, apparatus and system for efficient supply of ammonia from urea that does not have low-temperature penalties.